1. Field of the Invention
The present invention relates generally to a cellular radio communication system and, in particular, to a system access method of a narrowband terminal in a cellular radio communication system supporting both wideband and narrowband terminals.
2. Description of the Related Art
Long Term Evolution (LTE) utilizes Orthogonal Frequency Division Multiplexing (OFDM) in a downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in an uplink. Such a multiple access technique is characterized in that time-frequency resources carrying data or control information are arranged orthogonally to discriminate among per-user data and/or control information.
FIG. 1 illustrates a basic structure of time-frequency grid of radio resource for transmitting data and control channels in a downlink of a conventional LTE system.
In FIG. 1, the horizontal axis denotes time, and the vertical axis denotes frequency. An OFDM symbol is the smallest transmission unit on the time axis, a slot 106 includes Nsym OFDM symbols 102, and a subframe 105 includes two slots. A slot is 0.5 ms, and a subframe is 1.0 ms. A subcarrier is the smallest transmission unit in the frequency domain, and the entire system transmission band includes NBW subcarriers 104.
In the time-frequency grid, a Resource Element (RE) 112 is the basic unit indicated by an OFDM symbol index and a subcarrier index. The Resource Block (RB) or Physical Resource Block (PHB) 108 includes the Nsymb consecutive OFDM symbols in the time domain 102 and the NRB consecutive subcarriers in the frequency domain 110. Accordingly, an RB 108 includes Nsymb×NRB REs. An RB is the smallest unit that can be scheduled for transmission. In an LTE system, Nsymb=7 and NRB=12, and NBW and NRB is in proportion to the system bandwidth.
The control information is transmitted in N OFDM symbols at the beginning of the subframe. Typically, N={1, 2, 3}. Accordingly, the amount of control information carried in a current subframe depends on the value of N. The control information includes an indicator that indicates the number of OFDM symbols carrying the control information, uplink and downlink scheduling information, Hybrid Automatic Repeat reQuest (HARQ) Acknowledgement (ACK)/Negative ACK (NACK) signal, Multiple Input Multiple Output (MIMO)-related control information, etc.
The LTE system utilizes HARQ for retransmitting data that fails decoding in the physical layer. HARQ is a technique for ensuring reliability of data transmission in such a way that a receiver transmits a NACK to a transmitter to request a retransmission of the data that has failed decoding in the physical layer. The receiver combines the retransmitted data with the previously transmitted data to increase the data reception performance. If data is decoded successfully, the receiver transmits an ACK to the transmitter such that the transmitter can transmit next data.
In a cellular radio communication system, scalable bandwidth is a significant feature for providing various types of data services. LTE supports scalable bandwidths of 1.4, 3, 5, 10, 15, and 20 MHz. These bandwidths correspond to 6, 15, 25, 50, 75, and 100 RBs, respectively. The mobile carriers use one of the available bandwidths to provided services. However, it is then necessary for the LTE system having a specific system bandwidth to support the terminals with different bandwidth capabilities.
For example, an LTE system having a 10 MHz system bandwidth cannot simultaneously support a 10 MHz terminal and a 1.4 MHz terminal. Basically, the terminal supporting the bandwidth that is narrower than the system bandwidth cannot receive a downlink control channel that is transmitted across the entire system bandwidth in the LTE system.
Also, terminals supporting different bandwidths may interfere with each other.